We will measure in vitro thymidine-3H uptake by circulating leukocytes periodically during the course of chronic myelocytic leukemia and chronic lymphocytic leukemia. Results will be correlated with clinical and hematologic data to determine whether leukocyte thymidine uptake can serve as a sensitive index of the quality of disease control and predict response or resistance to chemotherapy in advance of other parameters. Prognostic value of such determinations will be determined by reviewing the course of patients who have been followed 1-4 years after initial determinations of leukocyte thymidine uptake. Thymidine uptake by circulating leukocytes and by leukocytes in marrow aspirates will be compared among patients with chronic myelocytic leukemia in different stages of the disease.